Nextcloud is a lot different that most containers, because i have or can have the functionality of a lot of the combined.
Usually most of the "Apps" are 1 click install, some of them need external services to run, like Redis, Collabora, ElasticSearch.
Some need extra packages installed like pdlib.
This particular container is based on alpine which alpine does not have yet a stable pdlib package, it's still on testing, That's why LinuxServer team haven't included it yet.
You can however install the package, manually or by making a script to install it on every docker restart or every update of the docker.
But I would recommend be first more familiar with Nextcloud first.